The LCD comprises a LCD panel. A back light module needs to be arranged at the bottom of the LCD panel due to that the liquid crystal can not give out light. The back light module can be arranged in a directly-down mode or a side-light mode according to the installation locations of a light source. The back light module in a directly-down mode comprises a light guide plate and an optical sheet covered on the light transmission surface of the light guide plate, and the side surface of the light guide plate is provided with the light source. As shown in FIG. 1, in the prior art, the optical sheet 100 is usually positioned by riveted metal posts 200 on the frame 300 of the LCD, but the factors such as temperature changes, etc. resulting in thermal expansion and cold contraction are not taken into account, so the deformation or warping of the optical sheet 100 with uneven optical performance will be caused.